In related art, a speaker module includes a casing and speaker unit accommodated and fixed inside the casing, wherein a rear acoustic cavity is formed between the speaker unit and the casing, and the speaker unit includes a diaphragm which vibrates to generate sounds. Generally, the space at the front side of the vibrating diaphragm is in communication with the sounding hole of the speaker module to radiate sounds to outside, and the space at the rear side of the diaphragm is communicated with the rear acoustic cavity. Since there is a phase difference of 180° between the sounds generated at the front and back sides of the vibrating diaphragm, the spaces at the front and back sides of the vibrating diaphragm are disposed separately with an interval, in which the rear acoustic cavity is a sealed structure.
When the rear acoustic cavity is relatively large in volume, the speaker module has a good performance in low frequency, however, when the rear acoustic cavity is relatively large but the sounding hole in the speaker unit is relatively small, a significant effect of Helmholtz resonance is generated which has a negative influence on the sounding performance of the product. Therefore, it is necessary to improve the speaker module in the prior art to overcome the above defect.